A hydraulic clutch comprises hydraulically actuated friction elements The friction elements generally include a housing, a piston and a clutch apply cavity defined between the housing and one side of the piston. The hydraulic clutch can also include a plurality of alternating metal plates and friction material disks on another side of the piston. Hydraulic oil is pumped into and out of the cavity for causing engagements and disengagements of the hydraulic clutch. In a vehicle power train, a hydraulic clutch may be positioned between a crankshaft and a gearbox for transmitting driving torques from the crankshaft to the gearbox. In the case of an automatic transmission, the gearbox itself may also include hydraulic clutches for actuating the gearwheels and, therefore, for changing gear.
For example, US 2009/0105039 A1 discloses a method of operating a powertrain that includes an automatic transmission, an internal combustion engine and an electric machine. The method includes a step of monitoring fluid pressure of a hydraulic clutch by using a pressure control switch. The pressure control switch is connected to a transmission control module for calculating a flow rate and a clutch volume in response to the fluid pressure. Operations of the hydraulic clutch are controlled based upon the clutch fill volume. However, the disclosed method involves expensive components and complicated techniques for its implementation.